A volcano is a landform that results from magma (molten rock within the earth) erupting at the surface. The size and shape of a volcano reflect how often it erupts, the size and type of eruptions, and the composition of the magma it produces.
If asked to draw a volcano, most people will sketch a steep, cone-shaped mountain, usually with clouds billowing from the summit. This is one type, but some of the most explosive volcanoes are less obvious, and represented by large depressions that may be filled with water.
Although New Zealand’s active volcanoes look quite different from one another, they can be grouped into three main landform types:
Each of these has distinct landforms, and the violence and styles of eruptions are unique to each. These differences reflect the main type of magma erupted:
Molten rock is called magma when it is beneath the earth’s surface. When it is erupted and flows through a volcanic vent it is called lava. And when it is erupted explosively as shattered fragments hurled into the air it is called tephra (which includes volcanic ash, pumice and scoria).
Magmas contain almost all of earth’s known chemical elements, but typically they consist of just nine: silicon, oxygen, aluminium, magnesium, iron, calcium, sodium, potassium and titanium. Oxygen and silicon together are the most abundant elements, making up 48–76% by weight of most magmas. The chemistry of magma, especially silicon content, is important for influencing the way it erupts. Three main magma types, and resulting volcanic rocks, are identified on the basis of their chemical composition.
Whether or not an eruption is quiet or explosive depends on the gas content of the magma. When the gas content is low, magma extrudes at the surface as lava flows. Very fluid basalt lava can flow over long distances, whereas viscous rhyolite lava piles up around the vent, like toothpaste squeezed from a tube, to form steep-sided mounds called domes.
Volcanic ash is not ash in the sense of the remains of something that has been burnt. In volcanology it refers to the size of grains. Grains less than 2 millimetres across are called ash. Those from 2 to 64 millimetres are called lapilli (from the Latin word for little stones). Larger material is called a block if it is dense and angular, or a bomb if it is full of gas bubbles and partly rounded.
When magma is rich in gas, it can produce explosive eruptions. As magma rises to the surface, the drop in pressure causes its gases to expand violently like the foam that explodes out of a champagne bottle when first opened. Shattered magma and rock fragments (pyroclastic material or tephra) are carried violently into the air before settling back to the ground. The higher the material goes, the further from the volcano it will be carried by the wind, so the intensity of an eruption can be judged partly by the distance its eruption material is spread.
A second type of explosive eruption occurs when magma contacts water beneath the ground or at the surface (such as in a lake or the sea). The hot magma vaporises the water instantly, causing violent steam explosions. Such eruptions take place regularly when magma rises beneath Ruapehu’s crater lake.
New Zealand lies at the south-west end of a vast horseshoe-shaped zone of intense volcanism and earthquakes. This zone extends, essentially unbroken, around the margins of the Pacific Ocean – the so-called Pacific Ring of Fire.
This immense belt of volcanic and earthquake activity corresponds closely with the edge of the Pacific tectonic plate, and also coincides with some of the most densely populated regions on the planet. More than half of the world’s active volcanoes above sea level are found in this zone.
New Zealand sits astride the colliding edges of the Pacific and Australian plates. The occurrence of earthquakes, jagged mountain ranges and volcanoes through New Zealand, and the contrasting geology and landscapes of the North and South islands, can all be explained by the different ways in which the edges of the two colliding plates are interacting along the length of the country.
When tectonic plates collide generally one of two things happen: either one plate slides beneath the other, curving back down into the mantle (a process called subduction), or the edges of the plates are crumpled and forced up, forming a wide zone of mountain uplift. Subduction occurs where one of the interacting plates is thin and dense and is forced beneath thicker and more buoyant crust.
As the subducting plate slides down into the earth’s hot mantle, the crustal rocks are heated and water and other volatile elements are boiled off. The chemical effect of this water is to lower the melting point of rocks in the solid mantle above the subducting plate, allowing magma to form. This buoyant molten rock rises to the surface and erupts to form volcanoes such as Taranaki (Mt Egmont) and Ruapehu.
To the north-east of New Zealand, the Kermadec Ridge is a chain of mainly andesitic submarine volcanoes formed above the subduction zone by melting of the oceanic Australian Plate.
Close to New Zealand, the Australian Plate changes from oceanic to continental crust and the Kermadec Ridge merges into the Taupō Volcanic Zone. This is a narrow zone of rhyolite calderas, with groups of andesite volcanoes at its southern end – Ruapehu, Ngāuruhoe and Tongariro, and at its northern end – Mt Edgecumbe (Pūtauaki), Whale Island (Moutohorā) and White Island (Whakaari). Rhyolite magmas form from melting of the continental crust.
Mayor Island (Tūhua) lies 60 kilometres west of the Taupō Volcanic Zone. It is separate from all the other volcanoes and it ejects a rare magma type, unusually high in alkali elements (peralkaline), not found elsewhere in New Zealand. The magma chemistry causes some unusual minerals to form, and tephras from Mayor Island can be readily identified by their distinctive mineralogy.
The high alkali content is responsible for the widespread occurrence of obsidian, a glassy rock, which forms when viscous lava is rapidly chilled. Mayor Island obsidian was collected and widely traded by early Māori settlers. Because of its distinctive composition, obsidian from Mayor Island can be readily identified by chemical analysis.
Volcanoes are classified according to their state of activity, which changes as magma rises towards the surface.
These are ‘picture-postcard’ volcanoes – conical structures up to several hundred metres high, built by many eruptions of andesite lava over tens of thousands of years.
Eruptions at cone volcanoes can be explosive (forming tephra or pyroclastic deposits) or quiet (forming lava flows). The eruptions often switch between these two styles of activity during a single episode. Because of the layered sequences, or strata, of lavas and pyroclastic deposits, cone volcanoes are also called stratovolcanoes. Sometimes the term ‘composite cone’ is used to indicate their layered nature.
Eruptions from cone volcanoes are generally small to moderate, with small eruptions occurring every 10–50 years. Taranaki, Ruapehu, Tongariro (including Ngāuruhoe), and Whakaari (White Island) are New Zealand's largest and most frequently active cone volcanoes.
Models of volcanoes are often exhibited at school science fairs – almost always large cones, with smoke or chemicals to simulate eruptions. Unfortunately they hardly ever win prizes because they are too common.
The different shapes of these four cone volcanoes reflect variations in the size and type of eruption, the number of different vents that have erupted at each volcano, and sometimes partial collapse by landsliding of the cone during an eruption, earthquake, or storm.
The classical, almost symmetrical cone shape of Taranaki (Mt Egmont) is due to recent eruptions coming from one main vent at the summit. The multiple peaks of Tongariro volcano are due to many vents being active at about the same time, forming several overlapping cones. Ngāuruhoe dominates the landscape, but it is simply the most recently active vent that has erupted many times over the last 2,000 years.
Although the central cone is the most obvious part of a stratovolcano, most are surrounded by a gently sloping plain (ring plain) formed of landslide deposits from giant avalanches, debris flows called lahars, and tephras from explosive eruptions. A lahar may be associated with a volcanic eruption, but may also be caused by the collapse of the unstable steep slopes or melting of snow near the top of a volcanic mountain. Many cone volcanoes like Ruapehu have crater lakes. A large body of water at high altitude can result in unexpected and disastrous lahars and floods downstream, as happened in the 1953 Tangiwai disaster.
Rather than a single mountain or crater, a volcanic field is an area containing many small, isolated scoria cones and craters. Each cone or crater has formed usually in a single, small eruption of basalt magma, which may have lasted a few days to a few months at most.
Existing craters or cones in volcanic fields are unlikely to erupt again. Instead the next eruption will form a new cone of scoria or ash, or explosion crater or small lava flow. Sometimes craters may overlap within the same cone as can be seen at One Tree Hill (Maungakiekie) in Auckland. The Auckland volcanic field has about 48 cones and craters. The most recent eruption occurred about 1400 AD, forming Rangitoto Island.
Basalt volcanic fields are also found in Northland – for example, near Kaikohe, Bay of Islands, and Whāngārei – where there is a wide range of ages dating from about 60,000 years to about 10 million years. The Bombay–Pukekohe–Pokeno area of South Auckland contains an extinct field (0.5 million to 1.5 million years old), and basalt volcanoes in the western Waikato date from 1.5 million years to nearly 3 million years ago. Much older volcanic fields are found in the South Island, for example in Otago.
The fluid basalt magma means eruptions from volcanic fields are small, and mostly lava-forming. Most explosive eruptions are relatively weak, throwing scoria out to no more than several hundred metres from the vent. If the magma encounters water as it is rising to the surface, steam explosions may excavate wide shallow craters (such as occurred at Ōrākei Basin and Lake Pupuke in Auckland).
Even the smallest eruption from the Auckland volcanic field would cause major disruption to the city, and dust-size volcanic ash is likely to spread over a large area.
The large, deep craters known as calderas have extremely violent origins. They form when a vast amount of rhyolite magma, bubbling with gas, erupts explosively from a magma chamber that may be only a few kilometres beneath the ground. During these eruptions, so much magma is erupted that the chamber empties, leaving the ground above it unsupported. This area collapses, dropping like a piston, to form a wide, deep depression.
In places the caldera walls can be seen as steep cliffs, but many are difficult to observe in the landscape because they may be filled in with erupted material or covered by water.
Rhyolite calderas may be active for several hundred thousand years, but large eruptions are rare, often with thousands of years between events. Caldera collapse is not the only effect on the landscape from these large explosive eruptions. Huge quantities of pumice, ash and gas are pumped into the atmosphere, and through a combination of heat and momentum, a seething column of this material may rise to over 50 kilometres above the caldera. From this height, ash and especially aerosols – gases and tiny drops of acid – can spread around the globe, affecting the world’s climate for several years.
Closer to the caldera the landscape may be buried by metres of pumice. The most devastating process, however, occurs when this column of material falls back to earth like a fountain, then surges out in all directions from the caldera as a hurricane-like billowing flow of hot pumice, ash and gas. These pyroclastic flows or ‘density currents’ can travel over 100 kilometres at the speed of a racing car, leaving behind a layer of volcanic (pyroclastic) debris that might be more than 100 metres deep. Some flows are so hot (600–700°C) and thick that the ash and pumice fragments fuse back together, forming solid rock known as welded ignimbrite. Cooler and thinner pyroclastic flows form loose, comparatively soft (non-welded) ignimbrites.
Eruptions from rhyolite volcanoes are not always so catastrophic. A small amount of rhyolite magma may remain after a caldera eruption, which is exhausted of all gas and so can only ooze from the volcano slowly, often along the faults and fissures opened up by earlier caldera collapse.
The very high viscosity of rhyolite lava means that it will not flow far, and instead it piles up around the vent to form steep-sided domes. These domes are prominent landscape features. For example, Mokoia Island and Mt Ngongotahā are rhyolite lava domes erupted within Rotorua caldera, and Mt Tarawera is a collection of lava domes that erupted around 1314 AD within Okataina caldera.
There are two active calderas in the Taupō Volcanic Zone which have erupted frequently in the last 10,000 years:
The Okataina caldera includes the Tarawera volcano which erupted most recently in 1886 and about 1314 AD. There are also at least six older calderas, including Mangakino, Kapenga, Whakamaru, Reporoa, Rotorua and Maroa. Large explosive eruptions over the last two million years from this nested collection of rhyolite volcanoes has produced a huge volume of pyroclastic rock which has buried older vents. The products of these big rhyolite volcanoes form the extensive flat ignimbrite plateaus flanking the eastern and western sides of the volcanic zone.
Lake Taupō, the largest lake in New Zealand, owes its existence and shape to the caldera-forming eruptions of the Taupō volcano – the most frequently active rhyolite volcano in the world.
The enormous Ōruanui eruption, about 26,500 years ago, formed the 30-kilometre wide depression at the northern end of Lake Taupō. Since then there have been 28 separate eruptions, ranging greatly in size. The latest (Taupō) eruption occurred about 1,800 years ago, and its effects on the landscape are still visible today.
Radiocarbon dating indicates an uneven spacing of Taupo's eruptions, from decades to thousands of years apart. This makes it difficult to forecast when the next eruption will occur and how big it will be.
The Ōruanui eruption (about 26,500 years ago) covered much of the central North Island with ignimbrite, up to 200 metres deep. Ash fallout was spread by the wind over the entire North Island, much of the South Island, and a large area east of New Zealand, including the Chatham Islands. About 1,200 cubic kilometres of pumice and ash were rapidly ejected. This caused a large area of land to collapse, forming the caldera basin now filled by Lake Taupō.
The Ōruanui eruption was so enormous that it is hard to visualise. In only a few days or weeks it ejected enough material to construct three Ruapehu-sized cones.
After the eruption, the new lake gradually filled to a level 140 metres above the present lake. The lake broke out to the north, resulting in a huge flood. For several thousand years the Waikato River flowed northwards into the Hauraki Gulf, but it later changed its course to flow through the Hamilton lowlands to the Tasman sea.
The most recent major eruption of Taupō volcano took place in late summer–early autumn around 200 AD, from vents near Horomatangi Reefs (now submerged).
The eruption produced a towering ash column, resulting in tephra-fall deposits over a wide area from Hamilton to Gisborne. The airfall deposits were much thicker to the east of Taupō because the eruption column was blown in that direction by strong westerly winds.
The eruption column was followed by a devastating pyroclastic flow, blanketing a roughly circular area within 80 kilometres of Lake Taupō with ignimbrite, and destroying all life in its path. The ground-hugging pyroclastic flow appears to be one of the most powerful ever recorded, and was able to overtop Mt Tongariro and the Kaimanawa mountains, climbing 1,500 metres in a matter of minutes.
The outlet of Lake Taupō was again blocked during the eruption, and the lake level rose to 34 metres above its present height, forming a widespread terrace. The lake eventually broke out in a huge flood whose effects can be traced for over 200 kilometres downstream, and include boulder beds and buried forests.
The Kermadec section of the Pacific Ring of Fire is mostly submarine, extending over 1,400 kilometres between New Zealand and Tonga. Only the Kermadec Islands occur above sea level. Even these are the emergent caps of larger submarine volcanoes.
Seafloor mapping techniques are now routinely discovering the location and structure of these volcanoes. Scientists can interpret the type of volcanoes by dredging seafloor samples. Since about 2000, these studies have discovered over 40 volcanoes larger than 5 kilometres in diameter, with some as large as Ruapehu, and some as shallow as 60–80 metres below sea level. It is almost certain that other volcanoes have yet to be discovered along the Kermadec Ridge.
Submarine cone volcanoes form classic steep-sided seamount volcanoes, built with layers of lava flows and volcanic sediments. These volcanic sediments are typically formed by various explosive interactions of hot lava with cold sea water during eruptions or by subsequent collapse at some time after the eruption. Most of the cone volcanoes are basalt or basalt–andesite in composition.
A critical factor in the evolution of submarine stratovolcanoes is water depth. The pressure of the overlying water reduces the quantity and rate at which bubbles can form in the magma rising beneath the sea floor. In deep water, bubble growth is suppressed and the magma generally erupts effusively as pillow lavas and sheet flows. In shallow water, bubble growth is more rapid and vigorous, potentially leading to explosive and fragmenting eruptions that produce volcanic sediments. Many of the Kermadec stratovolcanoes, built between water depths between 500 and 2,000 metres, show this transition between effusive and explosive eruptions: pillow lavas form on the deeper flanks and volcanic sediments at the peak. This change is gradual over a range of depths.
Seafloor mapping has also shown a greater number and size of rhyolitic caldera volcanoes than previously supposed. The Kermadec islands of Macauley and Raoul were known to have phases of caldera volcanism, with the associated formation and dispersal of pumice. However, new mapping suggests that caldera volcanism is not only a common eruption style north of Raoul, but also occurs to the south. Typically the calderas are 3–10 kilometres in diameter (up to the size of Wellington Harbour) with the caldera walls having relief of up to 700–800 metres. Some calderas ‘overprint’ each other and hence show multiple eruptions. Pumice has been dredged from many of these calderas, and in a few cases entirely mantles the edifice.
How pumice forms beneath the ocean is an intriguing question. Many of these eruptions, especially in shallow water, were explosive with a column of hot pumice, ash and gas sheathed by steam from the surrounding sea water. The hot pumice is suddenly chilled, with steam condensing to water in the pumice holes, and the pumice then sinks to the sea floor. These pyroclastic columns may also collapse to produce submarine flows, cascading down the outer flanks of the newly formed caldera.
Observing submarine eruptions is very difficult. Eruptions have been recorded from Raoul Island since 1800, including eruption sequences in 1814, 1870, 1964, and 2006. Small temporary islands have also been recorded close to Raoul and Macauley islands within the last 150 years, representing small cones that have subsequently been washed away by waves. Aircraft and fishing boats have also reported sea-surface disturbances, including rising steam, above shallow volcanoes.
Ship-towed hydrophones and seismometers on Pacific islands have recorded eruption sequences from Monowai volcano since the 1970s. More recently, repeat multibeam surveys of Monowai volcano in 1998 and 2004 revealed collapse of the volcano crest by about 100 metres, followed by rapid cone reconstruction. Seismometers also recorded a large explosive eruption in this period in May 2002.
Tephra is a general term for all the fragmental material erupted explosively from a volcano – ranging from fine dust (called ash) to car-size blocks. It is a Greek word meaning 'ashes', originally used by Aristotle to describe a volcanic eruption in the Aeolian Islands near Sicily, about 300 BC.
North Island volcanoes have blasted huge volumes of tephra into the air, to be blown over northern New Zealand and in some cases far out to sea, for more than 1,000 kilometres. This volcanism has deposited layer upon layer of tephras over the landscape. The layers have helped volcanologists work out the history of volcanoes and the distribution of their far-reaching airborne products.
In many parts of the North Island, natural cliffs along terraces, river banks or at the coast, together with cuttings made during road construction or quarrying, reveal blanketing layers of tephra fallout from numerous eruptions. They drape the landscape on which they fall, generally following the contours of hills, terraces and valleys.
Tephra-fallout layers have two special features:
Once identified by geochemical analysis, a tephra layer provides a marker bed for an ‘instant’ in time, that instant being the time of eruption that produced the layer. In New Zealand and elsewhere many studies have used tephra layers as a dating tool, a science called tephrochronology.
How can one tephra layer be distinguished from another? Scientists use many methods to characterise or fingerprint each layer, both in the landscape and laboratory.
In the landscape, colour, thickness and position of the tephra in the sequence are important, and sometimes the type of pumice is useful in identifying it. For example, pumice from the Taupō eruption (about 200 AD) is usually cream-coloured and easy to crush, whereas pumice from the Kaharoa eruption (about 1314 AD) is white and hard to crush.
A tephra layer from a single eruption may be tens of metres thick near its source and coarse grained, but over 100 kilometres away it thins rapidly to only a few centimetres or millimetres of fine ash.
In the laboratory, the types of mineral grains (crystals) sometimes allow the tephra to be identified and matched to a source volcano. For example, a widespread tephra erupted from the Tūhua caldera (Mayor Island) about 7,000 years ago contains very unusual minerals. This tephra can be identified instantly with a microscope, even from just a few grains. Chemical analysis of volcanic glass in tephra layers is the most useful way of fingerprinting them.
The thickest tephra sections occur downwind of the Taupō Volcanic Zone, but much of the central part of the North Island has a tephra mantle up to several metres thick immediately beneath the land surface. Because of this, many North Island soils have been derived from tephra deposits rather than the underlying bedrock.
Some of the most complete tephra sequences have been found in lakes and bogs. Thin layers only a few millimetres thick may be preserved, whereas they are rapidly eroded on dry land. For example, cores from lakes near Hamilton revealed at least 46 tephra layers 2 to 120 millimetres thick, from seven North Island volcanoes (Taupō, Okataina, Tūhua, Taranaki, Tongariro, Ngāuruhoe and Ruapehu) over the last 20,000 years.
Similar studies from Auckland showed that scores of thin tephras from the same North Island volcanoes, including at least 43 tephras from Taranaki volcano, have rained out over the area for more than 70,000 years. Older tephra deposits, many metres thick, strongly weathered and clay-rich in many places, also occur in both these areas.
The frequent eruptions from the volcanoes of Tongariro National Park, and Taranaki have added small amounts of many nutrients to soils downwind from the volcano. For example, the 1995–96 Ruapehu eruptions added up to 1,500 kilograms per hectare of sulfur and other elements to large areas of land in the central North Island. Volcanic topdressing and a favourable climate is the reason why top-class carrots are grown at Ohakune.
The longest and most complete tephra records have been obtained from deep-sea drilling. Recently, 134 tephra layers, one nearly one metre thick, were found in cores from Leg 181 of the international Ocean Drilling Programme, around 700 kilometres east of the North Island. The layers record repeated large explosive eruptions from the Coromandel volcanic zone (Coromandel Peninsula–Tauranga area) from 2 to 12 million years ago, and then in the Taupō Volcanic Zone from about 2 million years ago.
The thick mantle of ignimbrites and tephra-fall layers forming the central North Island landscape is clear evidence of many large volcanic eruptions over the last few thousand years and earlier.
In 1920, Thomas Jaggar, Director of the Hawaii Volcano Observatory, reported on the need for volcano surveillance in New Zealand, but his recommendations were not implemented. However, his visit initiated detailed mapping of volcanic rocks in the Taupō Volcanic Zone by Leslie Grange from 1926 to 1929. Grange reiterated the need for close observation of volcanoes in this region, but no action was taken.
In December 1953 a lahar from the crater lake on Mt Ruapehu washed out a bridge across the Tangiwai River, resulting in the death of 151 people from a train crash. This disaster led to recognition that volcanoes likely to erupt again should be regularly monitored. Although an eruption cannot be prevented, monitoring should indicate increased activity before an eruption, allowing for evacuation of people and mitigating the effects on property.
In 2006 monitoring of natural hazards (including volcanoes) was the responsibility of GeoNet, a collaborative project between the Earthquake Commission, GNS Science, and the Foundation for Research, Science and Technology.
Before an eruption happens, magma moves up towards the ground surface. Volcanologists use several methods to detect the movement of magma:
Earthquakes usually provide the first sign of unrest. The seismic results are sent by radio telemetry to GeoNet headquarters and continuously monitored. Once increased activity is detected, more detailed observations are undertaken. Earthquakes caused by the rise of magma – identified as ‘volcanic earthquakes’ – can be distinguished from those arising from ground movement caused by tectonic (non-volcanic) processes.
Those volcanoes that have erupted within the last 10,000 years, especially those with multiple eruptions in that period, are the most likely to erupt again. Together with Taranaki (Mt Egmont) and the Tūhua caldera (Mayor Island), most of the active volcanic centres are in the Taupō Volcanic Zone, and include Ruapehu, Ngāuruhoe, Tongariro, Taupō caldera, Okataina caldera, Mt Edgecumbe (Pūtauaki) and Whakaari (White Island).
Taranaki (Mt Egmont) has erupted at least a dozen times since about 1300 AD, the most recent (Tahurangi) occurring probably around 1755. Taranaki has generated a series of lahars in the last few thousand years, as well as eruptions, and part of the volcano's summit probably collapsed between 1860 and 1897.
Mayor Island (Tūhua), 25 kilometres offshore in the western Bay of Plenty, has erupted several times in the last 10,000 years. A caldera-forming event about 7,000 years ago spread tephra over part of the North Island. The latest eruption, of lavas, took place possibly about 3,000 years ago.
Mt Edgecumbe (Pūtauaki), an andesitic cone volcano, was active about 3,200 years ago.
Small eruptions have occurred irregularly over a large area in the Auckland volcanic field, the latest being the formation of Rangitoto Island around 1400 AD. Past experience suggests that future eruptions in Auckland are likely to come from new vents rather than existing cones.
All these volcanoes are monitored by the existing seismic network, which should give early warning of renewed volcanic activity. In some cases, however, the period of warning before an eruption may be only a matter of a day or two (as in Auckland) but in others (such as Taupō) it may be prolonged for months or even years.
The Kermadec Ridge continues north-eastwards for over 1,400 kilometres from the Taupō Volcanic Zone as a chain of submarine volcanoes. Oceanographic research has shown evidence of recent volcanic or hydrothermal activity. Three volcanic islands occur towards the northern end of the ridge, and there is a seismograph on Raoul Island (900 kilometres north-east of New Zealand). In contrast to the volcanoes on or close to the North Island, there is little monitoring along most of the Kermadec Ridge.
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Price, Richard, and others. Volcanology of the Tongariro Crossing; a virtual field trip on CD-ROM. University of Waikato, 2003.
Williams, Karen. Volcanoes of the south wind – a field guide to the volcanoes and landscape of Tongariro National Park. Tūrangi: Tongariro National History Society, 2001.
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